Make TypeMask an interface and start hiding implementation details of FlatTypeMask.

sdk/lib/_internal/compiler/implementation/ssa/types.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/types.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/types.dart	(revision 22167)
+++ sdk/lib/_internal/compiler/implementation/ssa/types.dart	(working copy)
@@ -18,7 +18,7 @@
       return isNullable ? HType.NULL : HType.CONFLICTING;
     }
 
-    Element element = mask.base.element;
+    Element element = mask.topClass();
     JavaScriptBackend backend = compiler.backend;
     if (element == compiler.intClass || element == backend.jsIntClass) {

[kasperl, 2013/04/30 14:03:24]

Move these predicates into TypeMask, so you can ask
mask.containsOnlyInts(compiler)?

[ngeoffray, 2013/05/01 12:56:16]

Done.

       return isNullable ? HType.INTEGER_OR_NULL : HType.INTEGER;